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Theme 78

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SECTION 1. AERODYNAMICS OF LIFTING SURFACES THEME 7. AERODYNAMICS OF THE WING HIGH-LIFT DEVICES Swept wings of rather small area with an airfoil of rather small camber andrelative thickness are applied in modern aircraft with the purpose of flight speedincreasing. Such wings can not provide large lift on landing modes because of earlyflow stall. The problem of increasing lifting properties for modern wings at high anglesof attack for shortening of take-off and landing distance is very actual now. For thispurpose wings are equipped with special design elements which allow to increase thevalue of C ya max in the area of critical angles of attack α st . These elements working onmodes of takeoff, landing and maneuver are called wing high-lift devices. The set of effective high-lift devices applied in aircraft is wide enough (table 7.1).There distinguish rigid, jet, combination high-lift devices and high-lift devices based onthe boundary layer control (BLC). The high-lift devices are installed on the leading and trailing wing edges. Thehigh-lift devices of the wing trailing edge are realized by flaps of various types (Fig.7.1): simple flap, one-slotted flap, Fowler extension flap, double-slotted flap, plane flapetc. Flaps are applied to increase the lift of an airplane at keeping of its position(keeping the angle of attack). They are extended while taking off and landing. The liftgrows due to increase of wing camber. Extension flaps consisting of several sections are used on modern airplanes.Multi-section configuration allows bending the wing smoothly, and air jets streaming onthe upper surfaces of sections through slots, providing smooth continuous flow at highangles of sections deflection. The theoretical substantiation of multi-slotted flaps wasgiven byS. A. Chaplygin. Such flaps additionally increase lift due to the growth of wing area. 81

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Fig. 7.1. High-lift devices of the wing trailing edge: a) - flap ΔC yа h − l .dev . = 0 .7 δ flap = 30 o ; b) - one-slotted flap; c) - one-slotted extended flap ΔC yа h− l .dev . = 1.1 ; d) - double-slotted flap ΔC yа h − l .dev . = 1.4 ; e) - Fowler flap; f) - plane flap ΔC yа h− l .dev . = 0 .8 ÷ 0 .9 δ flap = 60 o . An angle between chords of main flap section in deflected and non-deflectedpositions is called flap setting δ flap . It is measured in a plane, perpendicular to axis ofrotation; δ flap > 0 if flap is deflected downwards. The flap are used not only for improvement of take-off and landingcharacteristics, but also for direct control of lift, rational redistribution of loading whicheffects a wing, and also for drag reduction. The high-lift devices of the wing leading edge are usually made as the deflectedslats (Fig. 7.2): movable slat, Krueger slat, deflecting nose etc. The slats are intended for prevention of premature flow stalling from wing. It isreached due to wing camber at the leading edge and jet blowing onto the upper wingsurface through a slot. An angle characterizing turn of coordinate system related with the slat at itsdeflection is called slat setting δ slat . The slat is the wing-shaped and locates along the wing leading edge. Atincreasing of angle α under the influence of sucking force the slat is put forward intooperative location. 82

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Lets consider the influence of high-lift devices deflection of the trailing edge onto structure of flow about the wing. Comparison of pressure factor C p distributions chordwise at non-deflected and extended flaps (fig. 7.5) shows, that the flap deflection causes an essential growth of rarefaction along total upper wing surface, and not just on its deflected part. The appreciable increase of overpressure is observed along the total lower surface. As a result the lift coefficient Fig. 7.5. Pressure factor distribution increases. along airfoil outline with flap and For effective realization of factor C ya without it increasing it is necessary to provide attachedflow about wing with the extended high-lift devices. As its known, this is promoted byboundary layer control (BLC) by increasing of kinetic energy of decelerated air layer(blown off) or its removal from the flow (suction) (Fig. 7.6). The change of dependenceof lift coefficient is similar to slat application (Fig. 7.4). The control system ofcirculation ΔC yа h − l .dev . = 0 .6 ÷ 0 .8 at C μ = 0 .3 , systems with flow blowing-off fromslot on a wing tail part (Fig. 7.7) and system of blower of wing surface by jets from theengine (Fig. 7.8) are also examples of jet high-lift devices. The intensity of blower(blowing-off) is characterized by a factor of momentum: kg ⋅ m msV j s s , Cμ = (7.1) q∞ S j N 2 2 ⋅m mwhere m s is the air consumption per second, V j is the jet speed, S j is the wing areamaintained by high-lift devices, q∞ is the dynamic pressure. 84

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Table 7.1. High-lift devices. Increase of Angle of High-lift devices maximum lift basic airfoil at Remarks max. lilt Controls boundary layer. Slight extra drag at high speeds. 40 % 20 °Slotted wing Controls boundary layer. Extra drag at high speeds. Nose-up pitching 50 % 20 ° moment.Fixed slat Controls boundary layer. Increases camber and area. Greater angles of 60 % 22 ° attack. Nose-up pitching moment.Movable slat More control of boundary layer. Increased camber and area. Pitching 75 % 25 ° moment can be neutralized.Slat and slottedfl Complicated mechanisms. The best combination for lift; treble slots may 120 % 28 °Slat and double- be used. Pitching moment can beslotted Fowler flap neutralized. Effect depends very much on details of arrangement. 80 % 16 °Blown flap Depends even more on angle and velocity of jet. 60 % ?Jet flap Note. Since the effects of these devices depend upon the shape of the basicairfoil, and the exact design of the devices themselves, the values given can only beconsidered as approximations. To simplify the diagram the airfoils and the flaps havebeen set at small angles, and not at the angles giving maximum lift. 88

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THEME 8. WING PROFILE DRAG The profile drag is the sum of surface- friction drag and drag of pressure causedby pressure redistribution along the streamlined surface due to viscosity influence(sometimes latter item is called form drag). It is necessary to mean that surface-friction drag is the main part of profile drag ofstreamlined bodies (therefore it is often considered that C xp ≈ C x fr ). This circumstanceis taken into account in approximate methods of C xp calculation. It is possible to adopt,that C xp does not depend on angles of attack in modes of attached flow and thencalculation of C xp is performed at α = 0 (small change of C xp on angles of attack istaken into account at definition of induced drag, having put an effective aspect ratioλ eff , or separate items at polar calculating). In range of Mach numbers less than 4 ...5all drag components (wave, induced, profile) can be determined separately from eachother. At that the wave and induced drag are well calculated without the account ofviscosity. However at M∞ ≥ 4 ...5 (zone of hypersonic speeds) there are effects ofviscous interaction, which cause the necessity of the account of viscosity and pressuremutual influence, that makes wave and profile drag inter-related. Below we shall consider the method of calculation for streamlined bodies atM∞ ≤ 4 ...5 (without the account of viscous interaction). The most widespread engineering method of C xp calculation is method CAGI.According to this method the profile drag is determined as surface-friction drag of a flatplate with introduction of correction multipliers which are taking into account anadditional part of drag from pressure forces. According to CAGI method the wingprofile drag is determined by the formula C xp = 2С f η c η м (8.1)where С f is the drag coefficient of friction of one side of a flat plate in a flow ofincompressible fluid at identical to wing: Reynolds number Re and position of a pointof laminar boundary layer transition into turbulent x t ; the factor double value takes into 89